/. Embryol. exp. Morph. 97 Supplement, 277-289 (1986)
Printed in Great Britain © The Company of Biologists Limited 1986
277
Nucleocytoplasmic interactions in the mouse embryo
JAMES McGRATH AND DAVOR SOLTER
The Wistar Institute, 36th Street at Spruce, Philadelphia, PA 19104, USA
INTRODUCTION
Fertilized mammalian ova consist of haploid genomes derived from both parents
and cytoplasmic components inherited largely from the female parent. These three
cellular compartments must successfully interact with each other and with their
environment for development to proceed. These interactions require the transposition of nuclear and cytoplasmic products between cellular compartments with
resultant alteration of gene transcription and the cytoplasmic expression of preformed or newly synthesized gene products. We have investigated nuclear/
cytoplasmic interactions in the mouse embryo via the microsurgical transfer of
nuclei and cytoplasm. Experiments have specifically examined the ability of nuclei
from later developmental stages or from a different species to support development, volume relationships between nuclear and cytoplasmic compartments, and
the nonequivalency of the maternal and paternal genomic contributions to
development.
The ability of egg cytoplasm to alter the function of a variety of embryonic and
adult nuclei and the ability of these nuclei to support development has been
extensively tested in nuclear transplantation investigations in amphibian embryos.
These studies have shown that (a) early embryonic nuclei can support complete
development (Briggs & King, 1952), (b) nuclei from progressively later developmental stages are less able to support development with serial transfer nuclei
undergoing characteristic, clone-specific, developmental arrest (King & Briggs,
1956; Subtelny, 1965), and (c) instances of extensive but incomplete development
have been achieved with differentiated adult nuclei (Gurdon, 1962; Gurdon &
Uehlinger, 1966; Laskey & Gurdon, 1970; Gurdon & Laskey, 1970).
The inability of differentiated nuclei to support complete development has been
explained alternatively by the irreversibility of differentiation or the inability of
the transplanted nuclei to revert to the rapidly dividing cleavage state without
sustaining lethal chromosomal damage. Evidence for the latter is supported by
cytological studies of amphibian nuclear transplant embryos (Briggs, Signoret &
Humphrey, 1964; DiBerardino & Hoffner, 1970).
Key words: nucleocytoplasmic interactions, mouse embryo transplantation, nuclear transfer,
interspecific, haploid, Thp mutation, parthenogenones
278
J. MCGRATH AND D. SOLTER
NUCLEAR TRANSPLANTATION IN MOUSE EMBRYOS
Initial attempts to introduce nuclei into mammalian embryos utilized sendai
virus to fuse somatic cells with oocytes or early-cleavage-stage embryos (Graham,
1971; Lin, Florence & Oh, 1973; Baranska & Koprowski, 1970). Evidence that
foreign nuclei could persist in the mammalian embryo was achieved when
Modliriski (1978) injected 8-cell nuclei with the T6 marker chromosome into
nonenucleated zygotes and demonstrated the presence of the marker chromosome
in resultant tetraploid blastocysts. In a subsequent investigation Modliriski (1981)
demonstrated the similar ability of inner cell mass (ICM) cell, but not trophectoderm, nuclei to contribute to tetraploid blastocysts when injected into nonenucleated zygotes.
A more rigorous test of the ability of foreign nuclei to support development is
their placement in enucleated cytoplasm. Although the removal of the zygote
pronuclei can be accomplished via micropipette penetration of the ovum plasma
membrane (Modliriski, 1975,1980; Markert & Petters, 1977; Hoppe & Illmensee,
1977; Illmensee & Hoppe, 1981), these embryos frequently disintegrate following
manipulation. The development of a karyoplast fusion method in which nuclei can
be removed or introduced into mouse embryos without cell membrane disruption
(McGrath & Solter, 1983a) has greatly facilitated mammalian nuclear transplantation studies. In our initial description of this technique, 69 pronuclei were
reciprocally transplanted between zygotes of mouse strains that differed in their
coat colour phenotype. After transfer of resultant blastocysts into pseudopregnant
females, ten progeny with the coat colour phenotype of the donor nuclei resulted,
seven of which survived to adulthood. This result compared favourably with the
number of progeny born to control embryos (5/34).
Having established that the nuclear transfer method was well tolerated by the
embryo, we tested nuclei from successive preimplantation stages (2-, 4-, 8-cell
embryos and ICM cells) for their ability to support in vitro development when
transplanted into enucleated zygotes (McGrath & Solter, 19836, 1984fl). Our
results showed that whereas 95% of enucleated zygotes receiving pronuclei
developed to the morula-blastocyst stages, only 19% of enucleated zygotes
receiving 2-cell-stage nuclei did so. Complete preimplantation development of
enucleated zygotes receiving 8-cell or ICM nuclei was not observed.
In the preceding experimental series, however, nuclear introduction occurred
during the latter half of the first cleavage division. Developmental failure of these
nuclear transfer embryos, therefore, may have resulted from an inadequate
exposure of the donor nuclei to cytoplasmic signals present for only a short period
of time following activation. It is interesting to note that Czolowska, Modliriski &
Tarkowski (1984) observed the swelling of thymocyte nuclei in activated ovum
cytoplasm to equal that of pronuclei when nuclear introduction coincided with
activation, but diminished with increasing time between activation and nuclear
introduction.
Nucleocytoplasmic interactions in mouse embryos
279
In order to permit earlier exposure of donor nuclei to activated ovum cytoplasm,
we therefore introduced 8- to 16-cell mouse embryo nuclear karyoplasts into
activated oocytes within 3h of activation. Nuclear introduction was accomplished
using either inactivated sendai virus or electrofusion (Kubiak & Tarkowski, 1985).
4-6 h after activation the newly visible maternal pronuclei were microsurgically
removed and the embryos permitted to develop in vitro. Our results (Table 1)
show that 74% of control activated oocytes receiving two zygote pronuclei
developed to the morula-blastocyst stages. In contrast, of i98 activated oocytes
receiving 8- to 16-cell-stage nuclei, 112 (57 %) never divided and 61 (31 %) divided
only once. Thus, despite the presence of these nuclei in the ovum cytoplasm for an
extended period of time, very few nuclear transplant embryos were able to
develop. Nevertheless, a small proportion (3%) of the manipulated embryos did
achieve the 8-cell, morula and blastocyst stages suggesting that in exceptional
instances complete preimplantation development may be supported by midcleavage nuclei. However, caution must be exercised in interpreting this result
since the donor origin of these nuclei was not confirmed. Future experiments will
attempt to demonstrate nuclear origin and define parameters that may increase the
frequency of successful nuclear transplant embryo development. A possible
approach to the latter would be to extend the time that donor nuclei reside in early
ovum cytoplasm by serially transplanting nuclei into activated oocytes on successive days. The adaptation of electrofusion to the mammalian embryo (Kubiak
& Takrowski, 1985), which should permit serial karyoplast fusions (not readily
performed with sendai virus-mediated fusions; McGrath & Solter, unpublished
observation), could facilitate such an investigation. Preliminary evidence demonstrating the ability to passage nuclei through several mouse embryo first cell cycles
has been obtained (McGrath & Solter, unpublished observations).
Recently, the fusion of 8-cell-stage sheep embryo blastomeres with half of the
cytoplasmic volume of activated oocytes has led to the birth of live progeny
(Willadsen, 1986). However, Willadsen (1981) had previously demonstrated that
single 8-cell-stage sheep embryo blastomeres without the addition of activated
ovum cytoplasm will, at a reduced frequency, also give rise to live progeny.
Therefore, whether the addition of activated ovum cytoplasm to single 8-cell
blastomeres results in significant nuclear reprogramming remains unanswered.
Nevertheless, these experiments underscore significant species differences between mammalian embryos. It is of interest to note that ultrastructural changes of
nucleoli consistent with active rRNA synthesis occurs in the mouse embryo at the
late 2-cell stage (Hillman & Tasca, 1969) but is not observed in sheep embryos
until the 16-cell stage (Calarco & McLaren, 1976). In addition, whereas single
8-cell-stage sheep embryo blastomeres will give rise to live progeny (Willadsen,
1981), single 8-cell-stage mouse embryo blastomeres do not complete development (Tarkowski & Wroblewska, 1967; Rossant, 1976), unless combined with
additional blastomeres (Kelly, 1975).
198
24
112 (57)*
2(8)
1-cell
61 (31)
2(8)
2-cell
13(7)
0
3-cell
6(3)
2(8)
4-cell
4(2)
0
8-cell
1 (0-5)
2(8)
Morula
1 (0-5)
16 (67)
Blastocyst
CO 2 and90%N 2 .
* Total number of embryos achieving developmental stage after 5 days of in vitro culture.
Oocytes were obtained from C57B16/J females that received an intraperitoneal injection of 5i.u. pregnant mares serum gonadotropin
followed 48 h later by an injection of 5 i.u. human chorionic gonadotropin (HCG). 16 h post-HCG injection, females were sacrificed and oocytes
were removed from the ampullary regions of excised oviducts. Cumulus cells were removed by a brief incubation in Whitten's medium which
contained 500 units bovine hyaluronidase (Sigma). Oocytes were washed in Hepes-buffered Whitten's medium (HWM) and activated by a 7 min
incubation in 7 % ethanol in H W M at room temperature (Cuthbertson, Whittingham & Cobbold, 1981; Kaufman, 1982).
Enucleation a n d t h e placement of pronuclear and 8- to 16-cell nuclear karyoplasts into the perivitelline space of oocytes was performed as
previously described (McGrath & Solter, 1983a, b). Karyoplasts were fused with oocytes using either inactivated sendai virus (McGrath & Solter,
1983a) or electrofusion (Reichert, Scheurich & Zimmerman, 1981) as modified for the mouse embryo by Kubiak & Tarkowski (1985).
Electrofusion was performed o n a dissecting microscope with a pulse generator (Grass medical instruments) which delivered two 25 V pulses of
100 JUS duration. A t 4 - 6 h after activation the maternal pronucleus was removed from successfully fused karyoplast: oocyte pairs.
In preliminary experiments in which 8- to 16-cell-stage nuclei were randomly introduced into activated oocytes prior to second polar body
extrusion, difficulty was encountered in distinguishing the donor 8- to 16-cell-stage nuclei from the maternal pronucleus. Therefore, activated
oocytes were cultured in vitro and were used as recipients only after second polar body extrusion had occurred (45 min following activation).
Nuclei were introduced n o m o r e than 3 h after activation of oocytes. Introduction of the donor nuclei opposite the site of second polar body
extrusion permitted easy identification of the donor and maternal nuclei. Following microsurgery, embryos were washed and cultured for 5 days
in Whitten's medium (Whitten, 1971) as modified by Abramczuk, Solter & Koprowski (1977) under silicone oil in an atmosphere of 5 % O 2 , 5 %
8- to 16-cell nuclei
Zygote pronuclei
Total
Table 1. Ability of 8- to 16-cell-stage blastomere nuclei to support development when transferred into enucleated
activated mouse oocytes
g5
r
£jj
*
S
£L
jo
^
ffi
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Q
Q
Nucleocytoplasmic interactions in mouse embryos
281
INTERSPECIFIC NUCLEAR TRANSFERS
Preliminary results of transfers of both pronuclei between Mus musculus and
Mus caroli zygotes revealed developmental arrest of these embryos at or prior
to the 4-cell stage (Solter, Aronson, Gilbert & McGrath, 1985). The inability of
these interspecific nuclear transplant embryos to develop does not result from
the simultaneous transfer of membrane and/or cytoplasm in the pronuclear
karyoplast since control embryos that received only interspecific membranebound cytoplasm can develop to term (McGrath & Solter, unpublished observations). These preliminary data indicate that early embryonic development is
dependent upon correct nuclear/cytoplasmic interactions that may not efficiently
operate between disparate species. Future experiments will hopefully expand the
number and variety of interspecific transfers and examine the ability of such
embryos to develop when subjected to serial nuclear transplantations.
NUCLEAR/CYTOPLASMIC RATIOS: HAPLOID EMBRYO DEVELOPMENT
The above data demonstrate that in the mouse embryo successful development
is dependent upon nuclear/cytoplasmic compatibility. To determine whether
nuclear/cytoplasmic components exhibit quantitative interactions, we examined
the effect of the nuclear/cytoplasmic (N/C) ratio on haploid embryo development. Previous data have demonstrated that haploid embryos produced either by
ovum activation (Witkowska, 1973; Kaufman & Gardner, 1974; Kaufman &
Sachs, 1976) or by the microsurgical removal of a single pronucleus from fertilized
zygotes (Modlinski, 1975) develop poorly. Possible causes for this decreased
development include the expression of recessive mutations, decreased heterozygosity or an altered nuclear/cytoplasmic ratio (Kaufman & Sachs, 1976). We
have compared the developmental ability of haploid embryos produced by the
removal of a single male or female pronucleus to haploid embryos that underwent
normalization of their N/C ratio by removing a single pronucleus and additionally
half the cytoplasmic volume of the zygote. Our results (Table 2) show that an
increased proportion of these cytoplasm-depleted haploid embryos developed to
the morula-blastocyst stages and therefore a normalization of the N/C ratio in
haploid embryos results in improved, but not completely restored, development.
This result is consistent with the previous observation in which an increased
proportion of parthenogenetic haploid embryos that underwent immediate
cleavage (and thus normalized their N/C ratio) developed beyond the 4-cell stage
when compared to haploid embryos that possessed a decreased N/C ratio
(Kaufman & Sachs, 1976). In the latter investigation, however, the authors
concluded that the improved development of immediate cleavage embryos
resulted from the increased heterozygosity that occurs in immediate cleavage
embryos. We suggest improved development of these embryos occurs as a result of
a more normal N/C ratio. It is of interest to note that a greater proportion of
haploid embryos produced by zygote bisection (Tarkowski & Rossant, 1976;
282
J. MCGRATH AND D. SOLTER
Tarkowski, 1977) complete preimplantation development than haploid embryos
produced by pronuclear removal alone (Modliriski, 1975, 1980).
NONEQUIVALENCY OF THE MATERNAL AND PATERNAL GENOMES
Mammalian parthenogenones are inviable and generally undergo developmental arrest shortly after implantation. Development to the 25-somite stage has,
however, been observed (Kaufman, Barton & Surani, 1977). Generalized cell
lethality is not the cause of parthenogenetic inviability since parthenogenetic
contributions to the adult soma and germline are observed in parthenogenetic-wild-type chimaeras (Stevens, Varnum & Eicher, 1977; Surani, Barton &
Kaufman, 1977; Stevens, 1978). Proposed reasons for the death of parthenogenones have included the lack of an essential non-nuclear contribution by the
spermatozoan, the inability of the activating stimulus to recreate the stimulus
provided at fertilization, or homozygosity for recessive lethal mutations (Graham,
1974).
We and others have recently utilized nuclear transplantation to determine if
parthenogenetic inviability might result from differential functioning of the
maternal and paternal genomes during embryogenesis. In this proposal parthenogenetic embryos are inviable since they lack a paternal pronucleus, which, it is
suggested, possesses unique functions not shared by the maternal pronucleus.
Evidence that supports this proposal is summarized below.
Table 2. The effect of reduced cytoplasmic volume on the ability of haploid
androgenetic and gynogenetic embryos to develop in vitro
Total
Gynogenone
Androgenone
Gynogenonehalf cytoplasm
Androgenonehalf cytoplasm
Unmanipulated
control
1- to 3-cell 4- to 6-cell
8-cell
Morula
Blastocyst
75
74
57
52 (69)*
48 (65)
10 (18)
9(12)
16 (22)
16 (28)
3(4)
9(12)
8(14)
3(4)
0
6(11)
8(11)
1(1)
17 (30)
61
18 (30)
14 (23)
16 (26)
11 (18)
2(3)
2(2)
2(2)
10 (10)
91 (87)
105
0
* Furthest developmental stage achieved after 5 days of in vitro culture (%).
Fertilized zygotes were obtained from spontaneous C57B16/J inter se matings. Embryo
isolation and pronuclear removal were as previously described (McGrath & Solter, 1983a,
1984c). Removal of one half the cytoplasmic volume of the zygote was accomplished in a manner
essentially identical to that employed for pronuclear removal. Estimation of the volume of
cytoplasm removed was guided by the use of an ocular micrometer. Preliminary experiments
revealed that in some instances the mechanical stresses needed to remove half of the zygote
cytoplasmic volume resulted in the 'fusion' of the second polar body with the zygote. Therefore,
in all embryos in which the cytoplasmic volume was halved, the second polar body was
microsurgically removed prior to cytoplasm removal. Following microsurgery, embryos were
cultured in vitro as described in the legend to Table 1.
Nucleocytoplasmic interactions in mouse embryos
283
Nuclear transplantation of the Thp mutation
Inheritance of the Thp mutation, a deletion of the proximal portion of chromosome 17 in the mouse (Bennett etal. 1975; Silver, Artzt & Bennett, 1979), results in
viable progeny when inherited from the male parent, whereas inheritance of this
same mutation from the female parent results in embryonic lethality during the
latter half of embryogenesis (gestational days 15-21) or shortly after birth
(Johnson, 1974, 1975). We investigated the nuclear/cytoplasmic origin of maternal- Thp lethality by performing reciprocal pronuclear transplantations between
maternal-Thp and + / + zygotes (McGrath & Solter, 19846). Our results showed
that of 197 enucleated zygotes from Thp/+ females receiving + / + pronuclei, 45
normal-tailed progeny resulted. In contrast, of 206 + / + enucleated zygotes
receiving equal numbers of + / + and Thp/+ pronuclei, 16 normal-tailed and 2
short-tailed progeny resulted. Both short-tailed Thp/+ progeny died within 24 h of
parturition. We concluded that maternal-Thp lethality is nuclear in origin and
suggested differential functioning of the proximal portion of chromosome 17
during male versus female gametogenesis in the mouse.
Nuclear transfers between parthenogenones and fertilized embryos
Parthenogenone inviability has been investigated by Surani, Barton & Norris
(1984) by alternately introducing single male or female pronuclei into unfertilized
activated oocytes possessing a haploid maternal genome. These investigators
demonstrated that the introduction of a single male pronucleus restored complete
development to haploid parthenogenones whereas the similar addition of a single
female pronucleus resulted in early postimplantation lethality. In a similar investigation Mann & Loveil-Badge (1984) interchanged two maternal pronuclei
from diploid parthenogenones with the male and female pronuclei from fertilized
zygotes. Their results similarly showed that parthenogenetically activated cytoplasm could support complete development if it received a male and female
pronucleus but that fertilized zygote cytoplasm receiving two female pronuclei
underwent early postimplantation developmental arrest. These results demonstrate that parthenogenetic lethality does not result from a primary cytoplasmic
deficiency.
Fertilized androgenetic and gynogenetic embryos
Fertilized diploid gynogenetic embryos have been produced by suppression of
second polar body extrusion in fertilized embryos and the subsequent return of
these triploid embryos to the diploid state via the microsurgical removal of the
paternal pronucleus (Surani & Barton, 1983). Transfer of these gynogenones into
pseudopregnant females resulted in early postimplantation lethality and thus
inferred that parthenogenetic lethality does not result from a cytoplasmic
deficiency.
We have also investigated the ability of gynogenetic embryos to develop by
transplanting single pronuclei between fertilized zygotes. This experimental
284
J. MCGRATH AND D. SOLTER
Nucleocytoplasmic interactions in mouse embryos
285
design precludes any adverse effects of second polar body suppression and also
permits the formation of androgenetic nuclear transplant embryos (McGrath &
Solter, 1984c). Manipulated embryos were transferred into pseudopregnant
females and permitted to develop to term. Three progeny were obtained from 339
gynogenetic embryos and two progeny were obtained from 328 androgenetic
embryos. The phenotype of these five progeny, however, demonstrated that they
all possessed a maternal/paternal origin and thus resulted from the incorrect
assignment of the parental origin of the pronuclei at the time of nuclear
transplantation. In contrast, 18 progeny were obtained from 348 control nuclear
transplant embryos, in which a pronucleus was removed and replaced with a
pronucleus of identical parental origin, all of whom demonstrated a maternal and
paternal phenotype. A similar investigation (Barton, Surani & Norris, 1984) also
produced androgenetic, gynogenetic and control nuclear transplant embryos. In
this study, control embryos were similarly observed to develop to term whereas
gynogenetic and androgenetic embryos were observed to exhibit marked growth
retardation and malformations during early postimplantation development. These
authors additionally noted that gynogenetic embryos demonstrated a deficiency of
extraembryonic tissues. In contrast, androgenetic embryos possessed relatively
intact extraembryonic membranes but incurred a disproportionate retardation in
the development of embryonic structures.
In addition to nuclear transplantation investigations, two additional systems
have demonstrated differential functioning of the maternal and paternal genomes
during embryogenesis. The paternal X chromosome has been shown to undergo
preferential X-inactivation in murine extraembryonic tissues (Takagi & Sasaki,
1975; West, Frels, Chapman & Papaioannou, 1977; Harper, Fosten & Monk,
1982). Additionally, genetic analysis of the products of meiotic adjacent-2 disjunction in the mouse have revealed functional differences in the maternal/
paternal contributions to the embryonic genome (Lyon & Glenister, 1977; Searle
& Beechey, 1978; Cattanach & Kirk, 1985). In the latter investigations translocation heterozygotes were utilized to generate gametes that possessed a parental
duplication or deficiency of a single chromosome or chromosomal region. The
union of two gametes possessing complementary duplications/deficiencies results
in euploid embryos which inherit both copies of a chromosome or chromosomal
region from a single parent. These studies have mapped specific regions of the
genome that result in embryonic lethality or a distinct phenotype when inherited
solely from one parent. In such an investigation Cattanach & Kirk (1985) have
recently demonstrated that mice that inherit both their 11th chromosomes from
their maternal parent have a decreased body size whereas mice that inherit this
chromosome solely from their paternal parent possess an increased body size. The
Fig. 1. In vitro outgrowth of control (A-C), gynogenetic (D-F) and androgenetic
(G-I) blastocysts on the second (left), fourth (middle) and sixth (right) day following
transfer of blastocysts to Dulbecco's modified Eagle's medium. Embryos were
observed using a Zeiss inverted-phase-contrast microscope.
286
J. MCGRATH AND D. SOLTER
demonstration that this opposite phenotypic difference persists into adulthood is
of particular interest.
The preceding evidence demonstrates that functional differences between the
maternal and paternal gametic genomes can result in phenotypic differences in
early postimplantation embryos (Barton et al. 1984), gestational day 15-21
embryos (McGrath & Solter, 1984/?) and adult mice (Cattanach & Kirk, 1985). We
have attempted to determine whether such differences can be detected during
preimplantation development or in vitro postimplantation development by comparing the ability of androgenetic, gynogenetic and control nuclear transplant
embryos to develop in vitro. Of 69 gynogenetic embryos, 58 (84%) developed to
the blastocyst stage whereas only 29 of 69 androgenetic embryos (42%) reached
this developmental stage. The decreased development of androgenetic embryos
would appear to be only partially explained by the presence of lethal YY embryos
since this class of embryos should comprise only 25 % of the androgenetic embryo
population. Preimplantation development was not adversely affected by technical
manipulations in this experimental series since 45 of 48 (94 %) control nuclear
transplant embryos developed to the blastocyst stage.
The ability of androgenetic and gynogenetic and control blastocysts to undergo
in vitro postimplantation development was assessed by transferring these embryos
into Dulbecco's modified Eagle's medium. Androgenetic and gynogenetic embryos were observed to differ in two respects (Fig. 1). On day 2 of outgrowth 90 %
(28/31) of the gynogenetic, and all of the control (11/11) nuclear transplant
blastocysts, had initiated blastocyst outgrowth. In contrast, only 10% (2/20) of
day 2 androgenetic blastocysts had done so. Androgenetic embryo outgrowth was,
however, observed on the following day. Thus, androgenetic embryos were
observed to initiate blastocyst outgrowth approximately 1 day later than
gynogenetic and control embryos. On day 6 of outgrowth, 26 of 31 gynogenetic
embryos had degenerated and could no longer be observed and the remainder
consisted of only a few cells. In contrast, 18 of the 20 androgenetic outgrowths and
10 of 11 control outgrowths remained intact on day 6. The androgenetic and
control outgrowths were observed to undergo gradual degeneration during the
subsequent 5 days of in vitro culture. Therefore, there appears to be a selective
death of gynogenetic trophoblast cells in vitro, which parallels the paucity of
extraembryonic tissue observed in gynogenetic embryos in vivo (Barton et al.
1984).
The demonstration that maternal and paternal genomes are programmed to
function differently during development in mammals raises two possibilities. In
one, maternal and paternal genomes may have been programmed to function
differently during gametogenesis irrespective of each other or their shared
cytoplasmic environment. Alternatively, appropriate gene action may depend
upon intranuclear interactions so that activation of components of one parental
genome is dependent upon the presence of the opposing parental genome. No
evidence that would discriminate between these two possible mechanisms presently exists.
Nucleocytoplasmic interactions in mouse embryos
287
CONCLUSIONS
The mammalian embryo consists of haploid genomes inherited from the
respective parents and membrane/cytoplasmic components inherited largely from
the maternal parent. Successful development depends upon appropriate interaction of these three cellular compartments with each other and with their
environment. The results of altering the nuclear/cytoplasmic components of the
mouse embryo have revealed several important aspects of these reciprocal
interactions that include: (a) male and female gamete nuclei are functionally
distinct, (b) the newly formed embryonic nuclei interact with their cytoplasmic
environment in a stochiometric and species-specific manner and (c) as development proceeds cleavage-stage mouse embryo nuclei rapidly lose their ability to
support development when returned to zygote cytoplasm. Major goals of future
investigations will be to determine how and precisely when the maternal and
paternal gamete genomes are programmed to function differently and the mechanisms by which nuclear and cytoplasmic components mutually interact to result in
appropriate gene action.
This work was supported in part by grants HD-12487 and HD-17720 from the National
Institute of Child Health and Human Development and by grants CA-10815 and CA-25875 from
the National Cancer Institute. The authors wish to express their gratitude to Dr Mariette Austin
for critical reading of the manuscript and Ms Brenda Harling for excellent secretarial assistance.
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